Comprehensive effects of interdecadal change of sea surface temperature increase in the Indo-Pacific Ocean on the warming-wetting of the Qinghai–Tibet Plateau

The correlation characteristics between anomalous changes in summer precipitation on the Qinghai–Tibet Plateau (QTP) and the high-impact areas of global sea-surface temperature (SST) are mainly studied in this paper. The results show that the interdecadal change of the regional “warming-wetting” in China is especially prominent in the northern part of the main body of the QTP, which is therefore identified as the high-value area of precipitation variability. Investigations have revealed that the high-value areas of summer precipitation variability in the northern QTP are significantly correlated with four high-value areas of SST variability, namely the western North Pacific, the western Central Pacific, the Southwest Pacific, and the central Indian Ocean. In these four high-impact areas, a synchronous tendency is found in the SST increase and sea-surface specific humidity. Through the tracking analysis of the correlated vectors of the water vapor source for the warming-wetting of the QTP, it further confirms that the four high-value areas of SST variability in the Indo-Pacific Ocean are the major impact sources of water vapor transport for the warming-wetting of the QTP. Moreover, the comparison of the characteristics of various interdecadal global water vapor transport circulations show that from 1991 to 2020, the trans-equatorial water vapor transport from the Southern Hemisphere witnessed a notable increase, which furthermore suggests that the interdecadal change of SST increase in the Southwest Pacific and central Indian Ocean is the key reason for the warming-wetting of the QTP. In addition, a comprehensive image of high-impact marine water vapor sources for modulating the warming-wetting tendency in the QTP is proposed.

Under the background of global warming, a range of environmental issues are taking place in Asia, with those on the Qinghai-Tibet Plateau (QTP) especially prominent 1,2 . The QTP is one of the most sensitive regions to global warming and because of its special topography, the comprehensive effects of global warming has been amplified, thereby reinforcing the sea-land thermal contrast 3 . According to extensive observations and multiple key indices of current climate systems, the actual transpiration of the QTP has changed dramatically, exerting a considerable impact on the precipitation, lake water storage, and river runoff 4-6 over the Plateau. The Intergovernmental Panel on Climate Change (IPCC) pointed out in its Sixth Assessment Report that based on the current observation statistics and numerical simulation results, extreme weather events such as high temperature and heavy precipitation on the QTP are showing an increasing tendency in the context of global warming. Frequent extreme events of high temperature are likely to be associated with human activities while the association between the increasing number of extreme events of heavy precipitation and human activities is unclear yet 7  www.nature.com/scientificreports/ effects of the water vapor sources and reveal the water vapor transport "paths" caused by SST anomalies in the significant areas of SST increase: where g represents gravitational acceleration, u and v represent zonal and meridional winds, respectively, q represents the specific humidity, Ps represents the ground pressure level, P represents the atmospheric top pressure, and qu and qv represent zonal and meridional water vapor fluxes, respectively 38 . Next, the composite correlation vector is defined as: where − → R denotes the composite correlation vector, R u (x, y) denotes the correlation coefficient field of qu (i.e., the component of SST and zonal water vapor flux), and R v (x, y) denotes the correlation coefficient field of qv (i.e., the component of SST and meridional water vapor flux) 39 .
Other methods. In addition to the above methods, calculation methods such as anomaly, linear trend estimate, correlation coefficients, variable standardization, and multiple regressions were also employed in this paper 39 . And the annual average value from 1991 to 2020 was taken as the climate average value, i.e. the normal value.

Results
Variation tendency and variability distribution of summer precipitation in China. The characteristic of precipitation variability is an important index that reflects the regional response of the QTP to global climate change. In the context of global warming, summer precipitation over the QTP is affected by SST increases and west winds 31,40,41 , monsoons 42,43 , and structural changes in their circulations [44][45][46][47] . Meanwhile, the comprehensive land-air impact over the complicated megarelief results in a nonuniformity of the spatial distribution of climate change. The research results of our study reveal an opposite variation tendency of summertime precipitation variability in the south and north of the OTP from 1991 to 2020 (Fig. 1a). A decreasing tendency of precipitation was observed in the south and southeast of the QTP, while the central and the northeast parts of the QTP witnessed an increasing tendency. Hence the pattern of "dry south and wet north" is formed. This result is consistent with the analysis on the variation tendency of precipitation over the QTP by using the optimized variational TRMM precipitation products (Sun et al. 48 ). Based on the distribution characteristics of high value areas of precipitation variability, the high-value areas of the QTP are divided into two areas, namely area A (northern Tibet) and area B (Qinghai and northern Sichuan). The composite analysis of precipitation in the two areas indicates that the maximum values of precipitation variability in both area A and area B occur in the summer (Table S1). From the interdecadal perspective (Fig. S3), the summer precipitation showed a reducing trend in area A from 1961 to 1991, while significant rising trend was identified from 1991 to 2020. As for area B, relatively weak decreasing trend was found from 1966 to 1991, while 1991-2020 also saw significant increasing trend. So after 1991, the summer precipitation in both area A and area B exhibited significant uplifting tendency. Notably, the overall increasing tendency of the summer precipitation variability in the two high-value areas of QTP (area A and area B) shows salient variation scopes, obviously higher than that for mainland China ( Figure S2), suggesting that the warming-wetting phenomenon in China mainly occurs in the central and the northern part of the QTP, i.e., the high-value areas of precipitation variability (area A and area B).

Correlation characteristics of summer precipitation in high variability areas of the QTP and SST changes in high-impact areas.
On the temporal scale, the global average SST has increased in a nearly linear manner from the 1970s to the late twentieth century, consistent with the increase of greenhouse gases emitted by human activities, such as carbon dioxide. During this time, the 1980s and 1990s were two periods when the SST increased fastest 49 . Taking the Pacific and the Indian Ocean as key research objects, the 1991-2020 global SST variabilities were analysed (Fig. 1b) in this paper. It is found that four high-value areas of SST variability exist in the Pacific and the Indian Ocean, which are located in the western North Pacific (SST area 1), western Central Pacific (SST area 2), Southwest Pacific (SST area 3), and central Indian Ocean (SST area 4). Summer precipitation in the QTP's key areas of warming-wetting shows a synchronous interannual increasing trend with the SSTs in the four high-value areas of SST variability. Their correlation coefficients reach 0.40, 0.49, 0.61, and 0.49 respectively (values over the 90% confidence level based on the student t-test) (Fig. 1c). Next, a correlation analysis was conducted on the summer precipitation of anomalous warm-wet key areas (area A and area B) on the QTP and the global SST in the corresponding period to identify the relevant high-value areas (Fig. 1d); the four key areas we identified agree with the spatial distribution of high-value areas of SST variability (Fig. 1b). Such analyses demonstrate a probable salient correlation between the significant areas of global SST increase and the warming-wetting phenomenon of the QTP, meaning the significant areas are likely to be the four SST high-impact areas for the warming-wetting on the QTP. www.nature.com/scientificreports/

Characteristics of the synchronous variation trends between SST increase and sea-surface water vapor in high-impact areas in summer.
The sea-surface temperature anomalies in the Pacific and the Indian Ocean have a significant impact on the water vapor budget of the QTP [50][51][52][53] . Multiscale modulations of monsoons and atmospheric circulations triggered by the sea-land thermal contrast in summer constitute the anomalous water vapor transport structure of the ocean water vapor sources, generating a climate teleconnection with the atmospheric water cycle over the QTP or the land. Therefore, SST increases in the high-impact areas of the Pacific and the Indian Ocean in summer might result in local water vapor anomalies, and inter-hemispheric teleconnection water vapor transport might be one of the vital factors that regulates precipitation on the QTP. In light of the above considerations, the global sea-surface specific humidity variability from 1991 to 2020 ( Fig. 2a) was calculated. By comparing the four SST high-impact areas indicated in Fig. 1d, namely, the western North Pacific (the correlated area 1), the western Central Pacific (the correlated area 2), the Southwest Pacific (the correlated area 3), and the central Indian Ocean (the correlated area 4), it is found that all of them show high sea-surface specific humidity variability. Moreover, the sea-surface specific humidity variabilities in the four high-impact areas all demonstrate synchronous upward interannual trends ( Fig. S4a-d). The correlation coefficients of the SST and the sea-surface specific humidity in the four high-impact areas are 0.90, 0.52, 0.85, and 0.77 respectively (values over the 90% confidence level based on the student t-test) ( Fig. 2b-e). This result reveals that SST increases in the SST high-impact areas of the Pacific and the Indian Ocean in summer might lead to a tendency of anomalous high specific humidity in the local sea surface.
How does the anomalous water vapor flow pattern caused by the SST increase in the high-impact areas affect the warming-wetting of the Plateau? The QTP can capture anomalous warm and humid gas flows from the Indian Ocean, the South China Sea, the low-latitude western Pacific and other areas in the south through the "hollow heat island" effect and its continuous heat source 26 . The water vapor over the QTP has experienced dramatic changes under the effects of global warming, thus affecting the precipitation, lake water storage, and water vapor budget of the Plateau. According to the study by Zhang et al. 31 , increased water vapor in the west of the QTP in recent years has been the culprit for the precipitation growth in the central and western QTP. The stronger water vapor transport from the Indian Ocean to the Plateau boosts the www.nature.com/scientificreports/ water cycle on the QTP, which is the main process of water vapor for the "wetting" of the QTP. In this paper, the correlation vector method for water vapor transport is used as a tracer for the water vapor source to reveal any anomalous changes in the water vapor transport path due to SST anomalies in the high-impact areas. The aim here is to explore the regulating effect of the high-impact areas of SST increase on the water vapor transport in the cloud precipitation process on the QTP and further ascertain the structural characteristics of the water vapor transport channel correlated to the SST anomalies of the Pacific and Indian Ocean. According to Fig. 3a-d, the www.nature.com/scientificreports/ SST in area 1 and the whole-layer water vapor flux of East Asia over the same period demonstrate an anticyclonic circulation pattern, where a correlated water vapor flow A comes from the Pacific Ocean to the west and then turns northward to a high-value area of precipitation variability in North China and the northern QTP (Fig. 3a). The SST in area 2 and the whole-layer water vapor flux of East Asia over the same period demonstrate an anti- www.nature.com/scientificreports/ cyclonic circulation pattern in the Pacific, and the related water vapor flow B in the southwest is transported northward from the equatorial Pacific and turns westward to the high-value area of precipitation variability in the northern QTP (Fig. 3b). The SST in area 3 and the whole-layer water vapor flux of East Asia over the same period also demonstrate an anticyclonic circulation pattern in the Pacific Ocean, and the related water vapor flow C in the south crosses the equator, turns northward, and is transported to a high-value area of precipitation variability in the northern QTP (Fig. 3c). Again, the SST in area 4 and the water vapor flux of East Asia over the same period demonstrate an anticyclone circulation pattern in the central Indian Ocean, which is just on the southwest edge of the Pacific anticyclonic westward-extending circulation. After it crosses the equator, the related water vapor flow D is transported from the western QTP to a high-value area of precipitation variability in the northern Plateau (Fig. 3d).
Relative contribution of the summertime SST increase in the high-impact areas to the warming-wetting of the QTP and an image of its comprehensive effect. Based on the previous discussions, the western North Pacific, the western Central Pacific, the Southwest Pacific and the central Indian Ocean are identified to be the key areas affecting the summer warming-wetting of the northern QTP in China. Under the combined effects of the SST high-impact areas, it is worth noting which areas have the most significant impact on the warming-wetting of the Plateau and what the contribution of each SST high-impact area to the warming-wetting is. Therefore, the relative contribution of the SST in these high-impact areas in summer to the warming-wetting of the QTP during the same period is further quantified in this section. A standardized multiple linear regression equation with the standardized SSTs in the four high-impact areas as the independent variables and the standardized precipitation in the warming-wetting sensitive areas of the QTP as the dependent variable is established as follows: In Eq. (4), y represents the standardized precipitation in the warming-wetting sensitive areas of the QTP, and x 1 -x 4 denote the standardized SST (with x 1 for area 1: the western North Pacific; x 2 for area 2: the western central Pacific; x 3 for area 3: the Southwest Pacific; and x 4 for area 4: the central Indian Ocean).
The standardized regression coefficients can directly explain the share of contribution and configuration of the SST in the high-impact areas to the warming-wetting of the QTP under their combined effects. It can be learned from the relative contribution rates of the SST in the high-impact areas to the warming-wetting of the QTP that the Southwest Pacific is the most significant area for warming-wetting, with a relative contribution rate of 51%. The relative contribution of the central Indian Ocean is 24%. The impacts of the western North Pacific and the western central Pacific are secondary, with relative contribution rates of merely 12% and 13%, respectively. In summary, the trans-hemispheric SST increase, accompanied with energy and water vapor transport, can be determined to be a crucial and nonnegligible factor modulating the warming-wetting tendency of the QTP.
According to the Fifth Assessment Report by the IPCC, the global oceans are experiencing remarkable warming, with the fastest SST increase in the near-surface layer yet recorded 54 . Although global ocean warming is largely certain, there are great regional differences in the rate and magnitude of SST increase in time and space 55 . By comparing the anomaly field differences of the whole-layer water vapor fluxes from 1961 to 1990 and from 1991 to 2020, the differences in the trans-hemispheric ocean energy and water vapor transport circulation on interdecadal temporal scales under the influence of global warming are discussed in this paper to identify the large-scale circulation background for the formation of the warming-wetting of the QTP. It can be observed from the comparison between Fig. 3e,f that the circulation of the water vapor transport on the QTP affected by the anomaly field of the whole-layer water vapor fluxes from 1991 to 2020 was opposite to that from 1961 to 1990. That is, from 1961 to 1990, the water vapor transport fluxes from the Indian Ocean to the QTP and its East Asian region was shown as a north-south axial cyclonic circulation, with the southerly water vapor flow on its eastern side transported to the southern Plateau and the eastern region of China. In contrast, from 1991 to 2020, the water vapor transport fluxes of the Indian Ocean to the QTP and the East Asian region was exhibited as a north-south axial anticyclonic circulation. In this way, the southerly water vapor flow on its western side was transported to the western and central Plateau, while the northerly water vapor flow on its eastern side transported the Northwest Pacific water vapor flow from the east to the northern Plateau. Moreover, from 1961 to 1990, the anomaly field of the water vapor transport fluxes in the western Pacific was opposite to that from 1991 to 2020. Specifically, from 1991 to 2020, the anticyclonic circulation extended to the southern hemisphere, the trans-equatorial easterly water vapor flow on its southern edge passed through the Indian Ocean to the easterly flow of the southern edge of the anticyclonic circulation of the Plateau, and then was transported to the western and central part of the Plateau.
It reveals that the trans-equatorial water vapor transport in the Southern Hemisphere was significantly enhanced from 1991 to 2020, further indicating that the SST increase in the Southwest Pacific and the central Indian Ocean is the key reason for the warming-wetting of the QTP. In addition, the characteristics of the 500 hPa circulation situation were observed to be similar to those of the anomaly field of the whole-layer water vapor transport fluxes (Fig. S5a,b). The anticyclonic water vapor transport circulation in the Pacific and the Indian Ocean from 1991 to 2020 was favorable for the transport of warm-wet ocean water vapor to the western and central Plateau. Thereby, an image of the comprehensive effect of the water vapor transport structures in the four high-impact areas on the warming-wetting of the QTP is proposed, with the anomaly field of water vapor transport from 1991 to 2020 as the background (Fig. 3g).

Discussion
To focus on the trend impact of the "warming-wetting" of the QTP from 1991 to 2020, the correlation characteristics between the anomalous summer precipitation variation on the QTP and global SST high-impact areas were studied. The correlation mechanisms of the climate anomalous warming-wetting of the QTP and the changes in ocean water vapor sources of the Indian Ocean and Pacific were researched from the perspective of atmospheric water circulation so as to verify that the areas of the Pacific and the Indian Ocean linked to the Asian continent might be the impact sources of water vapor transport for the warming-wetting of the QTP. The reasons for the reinforced warming-wetting of the QTP in summer were analysed and the following conclusions were reached: (1) From 1991 to 2020, the regional characteristics of warming-wetting of the QTP varied significantly in summer. The precipitation variabilities in the southern and northern Plateau showed opposite trends. Specifically, precipitation in the south and southeast of the QTP showed a decreasing trend, while the precipitation in the central and the northeast of the QTP exhibited an increasing trend. Hence the pattern of "dry south and wet north" is formed on the QTP. (2) There were four high-value areas of SST variability in the Pacific and the Indian Ocean during the summers from 1991 to 2020, namely, the western North Pacific, the western Central Pacific, the Southwest Pacific, and the central Indian Ocean. Through the correlation analysis of the summer precipitation over the QTP's significant areas of warming and wetting (area A and area B) and the global SST during the same period, it is found that there are also four high-correlation areas in the Pacific Ocean and the Indian Ocean. And these four areas greatly coincide with the spatial distribution of the above four high-value areas of SST variability. (3) In summer, the spatial distribution characteristics of the high-value areas of SST variability are similar to those of the sea-surface specific humidity variability in the Pacific and the Indian Ocean. The SST increase and the sea-surface specific humidity in the four high-impact areas are featured by synchronous change trends. Their correlation coefficients reach 0.90, 0.52, 0.85 and 0.77, respectively (values over the 90% confidence level based on the student t-test). It is concluded that the SST increase in the high-impact areas of the Pacific and the Indian Ocean in summer can lead to enhanced regional sea-surface transpiration, which in turn results in the anomalous high humidity change trends of local sea-surface water vapor. (4) By analysing the correlated synthetic vectors between the SSTs in the four high-impact areas in the Pacific and the Indian Ocean and the whole-layer water vapor fluxes in East Asia during the same period in summer, it can be drawn that area 1 demonstrates an anticyclonic circulation pattern, where a correlated water vapor flow A, originally in the south flank of the area, comes from the Pacific Ocean to the west and then is transported from the southeast to the central and the northern QTP; area 2 is identified with an anticyclonic circulation pattern in the west of the central Pacific, and the related water vapor flow B comes from the equatorial Pacific, turns westward and then is transported from the southeast and the west to the central and the northern QTP; area 3 also demonstrates an anticyclonic circulation pattern in the southwest Pacific Ocean, and the related water vapor flow C crosses the equator, turns northward, and is transported from the southwest and the west to the central and northern QTP; area 4 is in the central Indian Ocean and just on the southwest edge of the Pacific anticyclonic westward-extending circulation. After it crosses the equator, the related water vapor flow D is transported from the western QTP to the central or northern QTP. (5) In summer, the contributions of the four high-impact areas of the SST increase to the warming-wetting of the QTP are significantly different. The contribution of each ocean impact source to the warming-wetting of the Plateau was identified as 51%, 24%, 12%, and 13%, respectively, by a standardized multiple linear regression analysis. Among them, the Southwest Pacific is the most significant area, suggesting that transhemispheric SST increases accompanied by energy and water vapor transport is a nonnegligible factor modulating the warming-wetting of the QTP. From 1991 to 2020, the trans-equatorial water vapor transport in the Southern Hemisphere was found to be significantly enhanced by comparing the characteristics of different interdecadal global water vapor transport circulation patterns. It illustrates that the SST increases in the Southwest Pacific and the central Indian Ocean are the primary causes for the warming-wetting of the QTP. The warming-wetting trend of the QTP can be modulated by trans-hemispheric water vapor transport and energy exchange in the above four high-impact areas with significant SST increases. Therefore, an image of the comprehensive effect of the high-impact oceanic water vapor sources modulating the warming-wetting trend of the QTP is proposed in this paper from the perspective of the interdecadal variability of summer monsoon circulation. (6) Does the phenomenon of warming and wetting over the QTP only exist in summer? By calculating the precipitation variation tendencies over the QTP in four seasons of spring, summer, autumn and winter from 1991 to 2020 (Figures S1), it is found that the spring precipitation over the QTP shows an increasing tendency, while in summer, opposite trends are observed between the south and the north QTP. As for autumn, an increasing trend is captured in the northeast and the north edge of the QTP, while the precipitation trend in the central and the south edge is the opposite. In winter, most part of the QTP exhibits a reducing trend in precipitation, but an upward trend is found both in the south edge and in the north edge of the QTP. The calculation of precipitation variabilities in area A and area B in four seasons shows that (Table S1) area A (central QTP) is accompanied with positive precipitation variabilities in spring and summer but negative values appear in autumn and winter. In area B (northeastern QTP), the precipitation variabilities are positive in spring, summer and autumn, and weak positive variability is observed in winter.
Notably, although the warming and wetting of the QTP signals strong regional characteristics in different seasons, the precipitation variation trend still displays the pattern of "dry south and wet north" from the www.nature.com/scientificreports/ perspective of the overall variation trend in four seasons in mainland China. This is likely affected by the high impact areas of SST that are related to the warming and wetting of the QTP. The seasonal variations of the water vapor circulation structures formed by the above high impact areas of SST may be correlated, to some extent, with the tendency of "dry south and wet north" in mainland China. The conclusion needs to be further studied in the future.

Data availability
All datasets used in this study are publicly available. The Sea Surface Temperature data are retrieved from https:// www. psl. noaa. gov/ data/ gridd ed/ data. cobe. html. The precipitation statistics obtained by 710 basic standard ground meteorological observation stations in the monthly value dataset of ground meteorological features in China (v3.0) provided by the National Meteorological Information Center of China Meteorological Administration (http:// data. cma. cn/). Reanalysis data provided by the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) (https:// psl. noaa. gov/ data/ gridd ed/ data. ncep. reana lysis. html).